1. Field of the Invention
The present invention relates to a low pressure sensor for sensing a change in the mass of a gas within a pressure vessel, and more particularly, to a temperature compensating low gas pressure sensor for use in a hybrid inflator of a vehicle safety restraint assembly.
2. Description of the Related Art
Numerous types of inflators have been disclosed in the prior art for expanding an inflatable air bag of a vehicle safety restraint system. One type of inflator utilizes a quantity of high pressure gas stored in a storage cylinder or body, which is selectively released to inflate the air bag. Another type of inflator derives the gas source from a combustible gas generating material, which, upon ignition, generates a quantity of hot gas for inflating the air bag. In still another type, the inflator includes both stored compressed gas and gas generating material for inflating the air bag. Such an inflator is referred to as a hybrid inflator, an example of which is disclosed in U.S. Pat. No. 5,360,232, assigned to the assignee of the present invention.
In a vehicle safety restraint system which partly or solely utilizes stored compressed gas it is very important to monitor the pressurized bottle containing the stored gas to detect any leakage in the container. If the gas pressure of the bottle falls below a predetermined level due to an undetected gas leak, the airbag effectiveness would degrade and the system will not operate properly.
Typically hybrid inflators are filled with compressed argon gas to approximately 3000 psi at 21.degree. C. However, this same volume of gas will diminish to a pressure of 2200 psi at -30.degree. C. and increase to a pressure of 4000 psi at 80.degree. C. Given that normal operating temperatures span such a wide range, compensation for temperature of the gas is a necessary function of a low pressure gas sensor. If such temperature compensation is not included in the low pressure gas sensor, the sensor would signal "low gas pressure" at cold temperatures and would not sense an actual low gas pressure at high temperatures.
It is known to merely detect loss of pressure in a pressurized vessel, with such loss being attributed to a leak in the vessel or other damage to the vessel. See U.S. Pat. Nos. 3,771,121, 4,049,935 and 5,296,659. U.S. Pat. No. 5,225,643, assigned to the assignee of the present invention, discloses a differential pressure switch disposed within a pressurized vessel.
A major disadvantage of the above differential pressure switches is the ineffectiveness for differentiating between a drop in pressure attributed to a change in temperature of the gas or a drop in pressure due to a leak.
U.S. Pat. Nos. 3,818,764 and 3,944,769 disclose pressure sensors which are temperature compensated by charging the sensor reference chambers with the same gas as the inflator. Thus, the switch must be pressurized and this pressurized gas may also leak. Moreover, continuous adjustment of the pressure is required.
U.S. Pat. No. 5,356,176 discloses a complex leakage detecting assembly which generates a signal in response to a change in temperature of the vessel through the use of a plurality of strain gauges and a layered bimetallic disk.